1. Field of the Invention
The present invention relates to a power output apparatus and a method of controlling the same. More specifically, the invention pertains to a power output apparatus for transmitting or utilizing power generated by an engine at a high efficiency and rotating a drive shaft in the reverse of the rotation of the output shaft of the engine. The invention also pertains to a method of controlling such a power output apparatus.
2. Description of the Prior Art
Torque converters utilizing a fluid are generally used to convert an output torque of an engine or the like to power and transmit the converted power. In the conventional fluid-based torque converters, an input shaft and an output shaft are not fully locked with each other and there is accordingly an energy loss corresponding to a slip occurring between the input shaft and the output shaft. The energy loss, which is consumed as a heat, is expressed as the product of the revolving speed difference between the input shaft and the output shaft and the torque transmitted at that time. In vehicles with such a torque converter mounted thereon, a large energy loss occurs in transient state like a starting time. The efficiency of power transmission is not 100% even in stationary driving. Compared with manual transmissions, the torque converters lead to a lower fuel consumption.
Some proposed power output apparatus do not use a fluid for torque conversion or power transmission unlike the conventional torque converters, but transmit power through mechanical-electrical-mechanical conversion. For example, a power output apparatus disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814 couples an output shaft of an engine with a rotating shaft of a d.c. motor via an electromagnetic coupling to make the rotating shaft work as a drive shaft. The engine drives one rotor on the side of d.c. field winding of the electromagnetic coupling, while the other rotor on the side of a.c. armature winding drives the rotating shaft of the d.c. motor or the drive shaft. Electric power produced by a slip between the two rotors of the electromagnetic coupling is supplied from the rotor on the side of a.c. armature winding to the d.c. motor via a rectifier. The d.c. motor also receives electric power from a battery to rotate the drive shaft. Unlike the conventional fluid-based torque converters, this proposed structure substantially has no energy loss due to the slip. It is accordingly possible to make the energy loss in the power transmission means relatively small by enhancing the efficiencies of the electromagnetic coupling and the d.c. motor.
In the proposed structure, however, the drive shaft (rotating shaft of the d.c. motor) is generally rotated only in the direction of rotation of the output shaft of the engine. No special account is taken into the requirement that the drive shaft is rotated in the reverse of the rotation of the output shaft of the engine.